Conservation and restoration of Cultural Heritage allows preserving and safeguarding monuments and art works for future generation. Among the causes which determine damage of artworks and Cultural Heritage, air pollution plays an important role, due to the increase in gaseous and solid pollutants concentrations in urban areas [1
] and, consequently, in museum environments [3
]. In general, buildings, monuments and products with heritage value require minimal and careful interventions based on surveys and diagnostic studies to find compatible materials [5
]. In particular, the choice of the restoration material is dependent on several factors: the kind of chemical interaction with the substrate, interface behavior, consistency and penetration capacity, workability, adequate mechanical properties, low creeping and shrinkage, and chemical and thermal resistance. Moreover, materials used for restoration applications, besides being compatible with the original material from the chemical, physical and mechanical point of view, should present similar aesthetic features, giving, at the same time, the opportunity to the restorer to highlight which kind of repairing action was made [5
]. Finally, the restoration material should be able to adapt to the masonry movements [6
] to avoid the risk of affecting the structural behavior of the monuments, causing collapse phenomena.
During the last decades, organic and inorganic synthetic materials have been developed to this aim focusing their application to different areas such as protective coatings, adhesive for different type of substrates (wood, wall, textiles, and ceramic), paintings, restoration and consolidation of masonry [8
]. In detail, innovative composite materials, such as fiber-reinforced polymers, steel-reinforced grouts and textile-reinforced mortars, have been widely employed in repairing and strengthening modern and historic buildings with structural purposes [7
]. Among these materials, great attention has been devoted up to now on carbon- and glass-reinforced polymers that have been widely used in the field of the Cultural Heritage thanks to their interesting mechanical and chemical properties (such as high tensile strength, stiffness-to-weight ratio, fatigue and corrosion resistance, and cost-effectiveness) of these materials. Unfortunately, the main drawbacks in the use of these systems are their brittle failure and sensitivity to impact, notching and environmental agents.
In this framework, a highly promising class of inorganic materials alternative to traditional binders are geopolymers [10
], which are amorphous materials obtained from the alkaline activation of an aluminosilicate source in a silicate solution. This reaction yields to a three-dimensional framework in which SiO4
tetrahedra are linked by corner-shared O atoms.
Geopolymers are characterized by interesting mechanical properties, low shrinkage, thermal stability, freeze–thaw, chemical and fire resistance, long term durability and recyclability. Thus, they can be used in place of Ordinary Portland Cement (OPC) in a wide range of applications, such as fireproof barriers, materials for high temperatures, matrices for hazardous waste stabilization, toolings and moldings [12
]. Besides, the use of geopolymer-based materials in concrete applications could significantly reduce the CO2
] thanks to the “low carbon” footprint of several raw materials with a high concentration of aluminosilicates from which they can be prepared, e.g., dehydroxylated kaolinite (metakaolin, MK) or industrial waste such as fly ash. Unfortunately, it should be pointed out that geopolymer slurries are characterized by poor rheological properties since they tend to drain due to a low viscosity that strongly limit their applicability in restoration works, in particular on vertical masonry or artefact.
Recently, to modify suitably the mechanical, physical and chemical properties of these systems, organic–geopolymer binders have been synthesized by means of a reaction of co-reticulation that takes place between the organic phase (epoxy resin precursors or polysiloxane oligomers) and the inorganic one [16
]. These materials have shown widely tunable performance depending on their composition and reaction conditions with significant potential applications in the fields of structural, fire-resistant and insulating applications [21
]. As far as their rheological behavior, it is worth pointing out that the addition of the organic resin into the geopolymeric slurry causes a significant change in the intrinsic viscosity of the whole system, thus allowing the obtainment of a homogeneous and workable thixotropic mixture [27
], easy to model in different shapes and to be spread also on vertical substrates.
The present paper reports on the preparation and characterization of geopolymer-based composites containing a limited content (up to 10% by weight) of a commercial epoxy resin and their use as potential repairing materials for different systems (tuff and cement-based materials). Moreover, to reduce the drying shrinkage of the material so favoring its adhesion to the investigated substrates, the composition of the geopolymer composites has been modified by addition of marble powder. The samples have been cured at room temperature to simulate outdoor conditions and subjected to morphological, thermal, rheological, physico-chemical and mechanical characterizations. In particular, aiming at the study of the compatibility, binding, protective and repair efficiency of the geopolymer-based composite with the substrates, detailed microstructural analyses and energy dispersive spectroscopy (EDS) mapping have been performed by means of scanning electron microscopy on the interfacial transition zone between the geopolymeric matrix and the substrates.
In addition, to consider the release of ionic compounds after the contact with aqueous solutions simulating the behavior in presence of atmospheric humidity, geopolymer samples were washed with ultrapure water and the obtained solutions were analyzed by determining pH and ionic composition.